This invention relates to mechanical assemblies of initially separate parts, and more particularly to mechanical assemblies which include hardened bearing surface elements secured to other members. For example, the invention may be applied to attaching a hardened bearing element to a piston (especially a master piston) in a compression release engine brake or to a cam follower in an internal combustion engine.
Although those skilled in the art will appreciate that the invention has many other applications, the invention will be fully understood from the following discussion which is chiefly in the context of applying hardened bearing surfaces to the master pistons in compression release engine brakes.
Meistrick et al. U.S. Pat. No. 4,706,625 (which is hereby incorporated by reference herein) shows a typical prior art engine brake. As shown in FIG. 1 of that patent, the typical engine brake includes hydraulic circuits, each of which (when the engine brake is on) transfers the motion of one part of the associated internal combustion engine (e.g., fuel injector rocker arm 72) to another part of the engine (e.g., sliding pin 51) to open an exhaust valve 60 in a cylinder which is at or near the end of its compression stroke. This releases the gas compressed by the piston in that cylinder and prevents the energy of compression from being recovered during the subsequent expansion stroke of the cylinder. Dissipation of energy in this way helps to slow down, retard, or brake the engine and the associated vehicle.
As shown in FIG. 1 of the '625 patent, each hydraulic circuit in an engine brake typically includes a master piston 66 and a slave piston 50. In the particular engine brake shown in '625 patent FIG. 1, adjusting screw 70 on rocker arm 72 bears on the lower surface of master piston 60 when the engine brake is on so that master piston 60 is driven to reciprocate by the arcuate reciprocation of the adjusting screw. The area of contact between adjusting screw 70 and master piston 66 is one of the highest stress regions in the engine brake. The top of the adjusting screw may be convex to avoid an edge of the screw contacting the master piston when the adjusting screw rocks due to oscillation of rocker arm 72. This convexity reduces the area of contact between the adjusting screw and the master piston, thereby contributing to the high stress in the contact area. The arcuate motion of adjusting screw 70 also causes the top of the screw to slide back and forth on the master piston surface it contacts, thereby increasing the tendency of the contacting surfaces to wear. To reduce this wear it has been necessary to ensure good lubrication between the master piston and the engine component such as adjusting screw 70 which contacts it. It has also been known to weld a layer of tungsten carbide material to the wear surface of the master piston. For example, such a tungsten carbide layer may be 0.014 to 0.015 inches thick.
Engine performance is constantly being pushed to higher levels. Increased engine performance leads to even higher stress on the master piston in an associated engine brake. This may cause the above-mentioned tungsten carbide layer to crack. Unfortunately, tungsten carbide can only be deposited on the master piston in a relatively thin layer as mentioned above. Engine manufacturers are also now interested in so-called "dry engines." These are engines employing much less lubricant than has been traditionally employed. Reduced lubricant reduces the horsepower required for lubricant pumping, and also reduces the amount of combusted lubricant (and therefore pollutants) in the engine exhaust. From the standpoint of engine brake manufacture, however, a dry engine is an even more severe operating condition because there is less lubricant available for lubricating the engine brake master piston surfaces which receive the driving inputs from the associated engine.
Substitution of a ceramic such as silicon nitride (Si.sub.3 N.sub.4) for the tungsten carbide on master pistons has been considered. This would be advantageous because silicon nitride has a modulus of elasticity close to that of the bearing steel (e.g., so-called 52100 bearing steel) typically used for parts such as the adjusting screw 70 which contact the master piston in FIG. 1 of the '625 patent. However, silicon nitride has a coefficient of thermal expansion which is much less than (e.g., only about one-sixth) the coefficient of thermal expansion of the steel used to make master pistons. This makes secure attachment of a ceramic element to a master piston difficult, especially in view of the fact that the ceramic element would typically be on the lower surface of the associated master piston, where it would be out of contact with any part of the engine when the engine brake is off.
In view of the foregoing, it is an object of this invention to facilitate the provision of mechanical components with securely attached ceramic bearing surface elements.
It is a more particular object of this invention to provide engine brake pistons (especially master pistons) with securely attached ceramic inserts for use as bearing surfaces.